1. Field of the Invention
This invention relates to an improvement in aerobrakes for space vehicles and more particularly, but not by way of limitation, to a multi-layer blanket insulation designed to function in a deployable/retractable aerobrake for a space vehicle.
2. Description of the Prior Art
It is known that multi-stage launch vehicles are the most efficient means to place payloads from Earth into high orbits. An approach is to consider current launch vehicles, with the exception of the U.S. Space Shuttle, are expendable assets, i.e. they are discarded after use.
The first stage or booster rocket typically expends at 70 to 100 nautical miles altitude. One or more upper stage rockets deliver the spacecraft or other payload to higher orbits or to an escape trajectory. The expended upper stages then burn up as they fall back to earth.
This is an expensive approach since complete launch vehicles cost tens to hundreds of millions of dollars. The upper stages may also cost from seven to seventy million dollars. Obviously this is expensive and will become even more so as space flight becomes more common. Thus, it becomes attractive to design launch vehicles for reuse.
A concept for an upper stage vehicle is the Space Transfer Vehicle ("STV"). This vehicle is designed to deliver payloads to geosynchronous earth orbit ("GEO") or higher. This vehicle will return to a low earth orbit ("LEO") for Space Shuttle or Space Station recovery, servicing, and reuse. Aerobraking appears to be the most cost effective and technically efficient means of recovering the vehicle to the desired orbit. It utilizes an aeromaneuver to dissipate excess velocity at LEO as opposed to carrying extra propellant and performing a retroburn.
However, aerobraking from GEO to LEO for recovery does present formidable technical challenges. To stay in a high orbit, an object must have high velocity (about 33,500 ft./sec. for GEO). To return and stay in a low orbit, an object must have lower energy (about 25,500 ft./sec. for a 160 nautical mile Space Shuttle orbit). Aerobraking is done by attaching a high aerodynamic drag device on a transfer vehicle returning from GEO, or other high energy mission and causing the vehicle to dip into the outer fringe of the Earth's atmosphere and to come out again one or more times. This path through the atmosphere or "aerocorridor" the vehicle must pass through is predetermined so that the high drag device (aerobrake) to slows the vehicle to around 25,500 ft./sec. as it emerges from its last pass through the atmosphere. This allows the STV to remain at a targeted LEO for rendezvous with a recovery Space Shuttle or a Space Station. Aerobrakes can be fixed or deployable. Aerobrakes will probably be in the range of 30 to 80 feet in diameter, so fixed brakes will probably be installed in space.
A deployable aerobrake must be capable of being contained within the payload envelope of the payload fairing or cargo bay of the launch vehicle to be folded to a stowed position and then deployed to an operating position. After braking the vehicle, the deployable aerobrake may possibly be subsequently retracted again to a stowed position to allow return to Earth. A successful aerobrake requires the use of a suitable thermal protection system.
The thermal protection system (TPS) used for the aerobrake is the single most important element in its design. Without a satisfactorily performing thermal protection system, the aerobrake cannot function properly. The major requirement for the TPS is that it must withstand the aerothermal healing environment encountered during the aeromaneuver without rupturing, tearing, or leaking or burning through. A desired requirement is that the brake be reuseable which, for a deployable aerobrake, implies that it must be fairly flexible both before and after encountering this heating.
Two major types of TPS are in current use for space vehicles. First is the rigid TPS of which two main subcategories are available. These are carbon-carbon (C-C) and fibrous refractory composite insulation. Because of the requirements on the retractable aerobrake for self deployment and retraction, a rigid type of TPS can be used only on small designated areas of this type of aerobrake. Of the two rigid types of TPS, the properties of carbon-carbon exceeded the temperature requirements but its cost is substantially higher than that of the fibrous refractory composite insulation. Carbon-carbon material is subject to oxidation and ablation. Fibrous refractory composite insulation can be made into tiles and is used extensively on the Space Shuttle. A fibrous refractory composite insulation tile coated with SiC can easily withstand all temperatures expected to experienced by the areobrake. Both rigid systems are about twice as heavy as flexible TPS.
The majority of the aerobraking surface for a deployable aerobrake must be covered by a flexible thermal protection system. NASA has developed several types of flexible insulation materials for the Space Shuttle program but all such materials are insufficient in terms of temperature limits. Recently NASA has been developing a new flexible thermal protection system known as Tailorable Advanced Blanket Insulation (TABI). It consists of a three dimensional woven refractory ceramic fabric with strips of insulating refractory felt running within the fabric. The upper, lower, and batten surfaces are made of this fabric. When this material is fabricated, all three fabric elements can be woven together in one process.
However, NASA's TABI is not suitable for the subject aerobrake because the folding concept for the contemplated aerobrake would require the TABI material to be folded in a direction that is perpendicular to the battens, which is in the blanket,'s stiffest direction. Also, the TABI material is somewhat porous. If an attempt were to be made to stop the porosity by coating the backside with an adhesive such a step would add unacceptable weight to the system, would require the web thicknesses to be doubled (to keep the backside temperature of the web below 550.degree. F.), and would decrease the flexibility of the aerobrake.
Thus, the present state of the art materials do not meet the requirements for this application of flexibility, low weight, low permeability porosity, and satisfactory temperature limits. It is believed that the present invention fully meets these requirements and makes practical the desired aerobrake for the STV.